Ideally, you want your extra payments to go towards the principal amount. However, many lenders will apply the extra payments to any interest accrued since your last payment and then apply anything left over to the principal amount. Other times, lenders may apply extra funds to next month's payment.
To get a payoff letter, ask your lender for an official payoff statement. Call or write to customer service or make the request online. While logged into your account, look for options to request or calculate a payoff amount, and provide details such as your desired payoff date.
How can I request a payoff statement? Contact your servicing bank (where you make your monthly mortgage payments) and request a payoff statement.